Molecular Rotors and Switches between Thermal and Electronic Excitation

• Applicants: Professorin Dr. Franziska Lissel; Dr. Francesca Moresco
• Subject Area: Experimental Condensed Matter Physics ; Synthesis and Properties of Functional Materials
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 545984433

Single molecule machines on a surface can convert external stimuli into motion. In the last years, several examples of rotations, translations, or conformational changes of molecules on a surface under the tip of a scanning tunneling microscope have been reported. However, the next step, i.e., developing mechanical molecular devices able to produce work or store energy, requires controlling the movement. For this, a detailed understanding of the underlying physical mechanisms is needed, which is still lacking. Thermal excitation can provide energy to the ground state of a molecule, yet according to the microscopic reversibility principle, unidirectional rotation is impossible in this case. On the other hand, tunneling electrons interact with the electronic excited states of the molecule on each electron transfer event and can allow directed, i.e., controlled motion. Both energy sources are available under the tip of a scanning tunneling microscope. In this project, we will combine molecular design and synthesis with scanning tunneling microscope experiments (imaging, spectroscopy, and manipulation) at variable temperatures to investigate molecular machines on a Au(111) surface and elucidate the physical mechanisms inducing controlled movements and conformational changes. Based on the established collaboration between the two participating groups, we will start with the synthesis of specifically designed vertical rotors and switches. These molecular machines contain two structural elements, i.e., an anchoring group for stable binding to the surface, and a switching or rotating part spatially decoupled from the surface. For the anchoring, we will employ N-heterocyclic carbenes, and combine them with molecular switches and rotors designed for having different energetic barrier heights for rotation. The vertically anchored molecular machines will then be tested using scanning tunneling microscopy. Specifically, we will combine surface thermal heating with tunneling electron excitation. The latter allows involving the electronic excited states of the molecule, a possibility which is absent in a classical case. The conformational changes of the switches, i.e., the toggeling between different states, will be induced by inelastic tunneling electrons and electric fields. The molecular rotors will be kept at a fixed temperature (varying from 5 K to RT) to investigate how thermal energy can be transferred to molecular mechanical degrees of freedom. Determining how the interplay between thermal energy and electron tunneling excitation contributes to movement will fundamentally advance the understanding of chemical reactions and mechanics on surfaces and provide unique information for the design of innovative molecular machines able to store thermal energy or produce work. In terms of quantum engineering, this opens fascinating perspectives in the direction of mono-thermal motors.

• DFG Programme: Research Grants